l82 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I44 



Ambulacra. — In the smallest specimen, the petals are short, extend- 

 ing less than one-half the distance from the apical system to the mar- 

 gin. In petal III there are ii pore pairs in the left poriferous zone, 

 and I less in the right; in petals II and IV, ii in the posterior porif- 

 erous zones with 4 less in the anterior ; in petals I and V, 14 in the 

 outside zones with 6 less in the inside. Because of the few pore pairs 

 in the petals, this difference in the number of pore pairs in the porifer- 

 ous zones of the same petal is very marked. In the larger specimens, 

 although the difference in the number of pore pairs remains approxi- 

 mately the same, it is less marked because of the greater length of 

 the petals. Pore pairs are added continuously throughout the growth 

 of the echinoid but at a slightly decreasing rate. In the specimen 

 21 mm. long there are 25 pore pairs in ambulacra III, in specimen 

 35 mm. long there are 36, and in a specimen 55 mm. long there are 43. 

 The plotting of these points on a graph produces a curve very similar 

 to that found in an ontogenetic study (Kier, 1957, p. 853) of Echino- 

 lampas fraasi De Loriol. The projection of this curve indicates that 

 the first pore pairs are probably introduced in individuals between 

 3 and 4 mm. long. Since there are more pore pairs in the outside 

 poriferous zones of ambulacra I and V, they would first occur in 

 these zones. The difference in the number of pore pairs in the 

 poriferous zones in the same petal is fairly consistent in all the speci- 

 mens, with I more in the left zone of petal III, except in one specimen 

 in which there is i more in the right; 2 to 4 more in the posterior 

 zones of petals II and IV, except for one specimen in which there 

 is I less pore pair in the posterior zone of petal II ; and 6 to 7 more 

 in the outside zones of petals I and V. 



The shape of the pores changes with growth. In the smallest 

 specimen, 8.5 mm. long, the pores are equal, both being round. 

 Mortensen (1948, p. 211) says of a specimen 12 mm. long that the 

 outer and inner pores "are, of course, not conjugate." However, in 

 this specimen which is even smaller than his, there is a definite con- 

 jugation groove joining the pores in the petals. In the specimen 

 21 mm. long (pi. 28, fig. i) the pores of a pair are no longer equal 

 as the outer pore is elongated transversely. In the specimens 35 mm. 

 long (pi. 28, fig. 2) and 55 mm. long (pi. 28, fig. 3) the outer pores 

 are increasingly more elongated and the conjugation groove more 

 marked. 



Periproct. — On the smallest specimen, the periproct is situated less 

 posteriorly than on the larger, being less than two-thirds the distance 

 from the apical system to the posterior margin, whereas on the largest 



